The present invention relates to injection molding fiber reinforced and non-reinforced resins especially in multi-cavity molds.
It is frequently desirable to oscillate the molten resin inside the cavity of a mold. Oscillation is particularly desirable in fiber reinforced resins in order to obtain uniform properties in the molded article. However, oscillation of the resin in the mold cavity is also desirable in non-reinforced resins in order to obtain the desired product uniformity.
The process of oscillating molten resin inside the cavity of a mold in order to orient the fibers or molecules within its structure is described in U.K. Patent 2170142 and in the article "Multiple Live-Feed Injection Moulding, Plastics and Rubber Processes and Applications", 7(1987) 3-10, By P.S. Allen and M.J. Bevis. The procedure described therein utilizes a manifold containing two pistons which is attached to the nozzle of the injection plasticizing unit. This splits the resin flow path into two channels which continue through a mold hot runner manifold to injection gates or orifices on opposite ends of the mold cavity for forming a molded article. If the resin is injected into the mold, the pistons operate in an alternate manner to effectively oscillate the resin inside the mold cavity in order to orient the fibers or molecules. European Published Patent Application 0,339,184 to Gutjahr shows a similar process; however, the oscillation of the resin in the mold cavity is performed by the injection-plasticizing units themselves. In this case, two such units are required and the resin is supplied to the mold directly through cold sprues and runners. It is a disadvantage of the Gutjahr procedure that premature freezing of the melt is more likely since cold runners are being used. Also, the Gutjahr process requires two injection-plasticizing units which involves additional cost.
A disadvantage of both of the foregoing procedures is the fact that they involve a single mold cavity. It is clearly desirable to utilize injection molding with oscillation in the mold cavity and with a plurality of mold cavities especially fed from a single feeding means or injection-plasticizing unit. The procedures of both of the foregoing references are not readily adaptable to use in a multi-cavity mold which is a significant disadvantage.
U.S. Pat. Nos. 4,717,324 and 4,863,369, both to Schad and Brown, show a hot runner system for a mold including shooting pots individual to each mold cavity for feeding precisely measured quantities of thermal plastic materials to each cavity.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for molding plastic articles wherein the plastic is oscillated in the mold cavity.
It is a further object of the present invention to provide a method and apparatus as aforesaid suitable for use with a plurality of mold cavities supplied from a single feeding means.
It is a further object of the present invention to provide a method and apparatus as aforesaid using a hot runner system.
Further objects and advantages of the present invention will appear hereinbelow.